Silica surface hydroxyl content

Figure 1. Surface hydroxyl content for selected silica gels...

Figure 3. Heat of immersion in water of silica gel surfaces as a function of surface hydroxyl content...

The immersion heat curves for the three gel samples are shown as a function of surface hydroxyl content in Figure 3. Silicas SB and FS exhibit a linear relationship between hydroxyl content and immersion heat over segments of the total curve. The rehydration of silica SL is confined to a very limited silanol content. Rehydration energies, obtained from the straight-line portion of the immersion heat curves, are given in Table I. 

Hydroxyl Population. All of these facts indicate a connection between the hydroxyl population on the silica surface and the catalyst s activity and relative termination rate. Figure 3 plots this decrease in the hydroxyl population. Silica, containing no chromium, was calcined at various temperatures and then reacted with CH3MgI solution. The amount of methane released was taken as an indication of the surface hydroxyl content. As the activation temperature was increased, the hydroxyl population decreased from over 4 OH/nm at 200 C to less than 1 OH/nm at 900 C. However, it never actually reached zero even at the highest temperatures studied, but was always significant compared to the coverage by chromium. 

As will be seen shortly, an analogous result is obtained with the silica-water system, where the BET monolayer capacity of water calculated from the water isotherm is roughly equal to the hydroxyl content of the silica surface. 

The relationship between the BET monolayer capacity of physically adsorbed water and the hydroxyl content of the surface of silica has been examined by Naono and his co-workers in a systematic study, following the earlier work by Morimoto. Samples of the starting material—a silica gel—were heated for 4 hours in vacuum at a succession of temperatures ranging from 25 to 1000°C, and the surface concentration of hydroxyl groups of each sample was obtained from the further loss on ignition at 1100°C combined with the BET-nitrogen area. Two complete water isotherms were determined at 20°C on each sample, and to ensure complete... 

In Table 5.3, is compared with the total hydroxyl concentration (Ni, + N ) of the corresponding fully hydroxylated, sample. The results clearly demonstrate that the physical adsorption is determined by the total hydroxyl content of the surface, showing the adsorption to be localized. It is useful to note that the BET monolayer capacity n JH2O) (= N ) of the water calculated from the water isotherm by the BET procedure corresponds to approximately 1 molecule of water per hydroxyl group, and so provides a convenient means of estimating the hydroxyl concentration on the surface. Since the adsorption is localized, n.(H20) does not, of course, denote a close-packed layer of water molecules. Indeed, the area occupied per molecule of water is determined by the structure of the silica, and is uJH2O) 20A ... 

A determination of the surface hydroxyl groups was made by Noll et al. (183). The total water content was determined by heating to 1100°. The content ofmolecular water was titrated by the Karl Fischer method. Silanol groups react with this reagent only very slowly. Good agreement was observed with silanol contents determined independently by other methods. With silica gel, 5.2 silanol groups were found per 100 A. ... 

For this complex, molecular chemistry does not adequately model the surface reactivity and the latter is strongly influenced by the presence of surface hydroxyl groups [22]. The organometallic fragments immobilized on silica have been reacted with trimethylphosphine to afford different silica-supported phosphine complexes of rhodium. The course of the reaction depends strongly on the hydroxyl content of the silica surface [23] (Scheme 7.2). 

The deposition of the silane in the loading step has been studied. The surface loading was correlated to the specific surface area of the silica rather than the hydroxyl content. Part of the deposition, therefore, is non-hydroxyl specific. The above reaction schemes, however, indicate the crucial role of the hydroxyls in the consolidation of the silane coating on the surface. 

A relatively easy way to check the existence of reaction (N) with strained siloxane bridges is to replace and/or to block all surface hydroxyl groups by a reaction with HMDS. This deactivated silica gel is then reacted with trichlorosilane. Kieselgel 60, thermally pretreated at 973 K, was refluxed with HMDS and consequently reacted with TCS at 623 K for 1 h. The Cl-contents on the surface of the silylated samples was determined. The results are presented in table 9.8. 

The catalyst was prepared by the reaction of Ti(0 Pr)4 (99.999%, Aldrich) with a non-porous, pyrogenic silica, as described previously (13). Degussa Aerosil -200 (surface area 183 mVg) was rehydrated then partially dehydroxylated in vacuo at 200°C, after which treatment the hydroxyl content is reproducible at 2.6 OH/nm (14). All subsequent manipulations were performed in situ in the absence of inert gases or solvents, using standard breakseal and high vacuum techniques. 

The polarization effects, isotopic effects (exchange of DjO for H2O), and non-ohmic behavior of adsorbed water on silica gel (surface area 660 mVg) are similar to those of an electrolyte solution [4]. The resistivity was found to decrease exponentially with water content and to increase roughly exponentially with OH content this was determined from a simultaneous measurement of resistivity and the near-infrared spectra of adsorbed water and surface hydroxyl groups. The adsorbed water on silica gel is immobile because the water molecule is hydrogen-bonded to two substrate hydroxyls, but the conductivity of silica gel increases as the amount of adsorbed water increases. 

Another way to preserve the monolithicity of polymeric gel is to replace alcohol with solutions having higher and higher water contents. This method achieves hydroxylation of silica surface particles in addition, an increase in network connectivity is expected, as in aging [62]. This reaction is temperature dependent. Treatment with acid water, which removes the residual Si-OR... 

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